Method of preparing barium titanate ceramics with flattened temperature characteristics



ragonal.

METHOD OF PREPARING BARIUM TITANATE. CERAMICS WITH FLATTENED TEMPERA- TURE CHARACTERISTICS Howard 1. Oshry, Erie, Pa., assignor to Eric ResistonCorporation, Erie, Pa., a corporation of Pennsylvania No Drawing. Application January9, 1953 Serial No. 330,584

10 Claims. (Cl. 106-3.9)

In m application Serial No. 224,389 filed Ma a.

1951, there are disclosed barium titanate ceramic dielectries in which the randomly oriented crystallites are sufficiently small, e. g. pigment or'sub-pigment sizes, to appear cubic below the Curiepoint by X-ray diffraction.

- ference in the piezo-electric properties due to the polarization of the barium titanate ceramic, the piezo-electric properties being markedly greater in the ceramic having large crystallites which appear tetragonal below the Curie point. a

e As disclosed in detail in the aforesaid application, the. fiattemperature characteristics (plus or minus 5% over the range minus v60 C. to plus 135 C.) are obtained by adding to calcined barium titanate powder whichhas a United States Patent 0 By having the crystallites small ice Patented Nov. 23, 1954 in the calcined powder appear cubic by X-ray diffraction.

There is no theoretical reason why all organic acids should not be usable. Oxalic acid, one of the strongest of the organic acids and which is readily available in high purity,

is preferable because his easily handled and the acid radical which must be driven off is not as large as in other organic acids such as acetic acid. 1

It is important that the barium titanate salt be in the crystalline state. For the oxalate, the precipitated salt is held in contact with the solution from which it is precipitated for about 16 hours. The range of from 4 to'72 hours has been investigated ,and the longer number of hours does no good. This step probably results in the growth of crystals to an optimum size .and this is accomplished by the 16-hour hold time andis no better accomplished by longer hold times; and since the l6-hour hold time is not unreasonably long, there is little advantage in choosing a lesser hold time.

The crystalline barium titanate salt (e. g. the oxalate) is then calcined at a temperature at which the acid radical is driven off and the barium titanate crystallites remaining after the acid radical is driven off grow in a form which appears cubic-below the Curie point by X-ray dift'rac-' tion. -Naturally, the higher the temperature the fasterthe calcining, but too higha temperature must be avoided as the crystallites will then appear tetragonal below the Curie point and the resultant calcined .barium titanate powder will be unsuitable for the purposes of this application. Calcining for 2 hoursat 1050 C. .results in the cubic crystallites. Longer times are necessary at lower temperatures, e. g. 4 hours at 950 C. The practical difliculty of controlling the temperature to prevent local-.

- ized over-heating would make shorter times at higher temparticle size small enough to appear cubic by X-ray' diffraction below the Curie point minute amounts of iron (less than 3% by'weight and preferably in the range of .l% to 1% and then firing the mixture to produce a dense ceramic. The 'fired ceramic will then have crystallites which appear cubic by X-ray diffraction below the Curie point and the reduction in the temperature constant of the dielectric constant and in the piezo-electric properties of the polarized ceramic will be exhibited.

This important difference inthe properties cannot be obtained if the iron additions are made to barium titanate in which the crystallites appear tetragonal below the Curie point. The iron must be mixed with barium titanate which appears cubic. After the iron addition, the iron inhibits the change from cubic to tetragonal which would occur in the absence of the iron.

The lower ranges of the iron additions do not completely flatten the temperature characteristics and the 'higher range decreases the resistivity so the ceramic becomes a semi-conductor.

Accordingly, the selection of the magnitude of the iron additions is to some extent a matter of judgment.

Iron, while perhaps the best,.is not the only addition. The same sort of effects are obtained by other metals of bivalent ionic radii and bivalent ionic potentialsv comparable to iron such as nickel, cobalt, magnesium calc um and manganese which have bivalent lOl'llC r-adu in the range of'0.6 to 1.0 Angstrom unit and bivalent ionic potentials in the range of 1.4 to 1.8. The requiredamount of these other metals is more than with iron (the median range being 3% to 1% as compared to. .25% to .75%

the change of the crystallites from the form which appears cubic below the. Curie point by X-ray diffraction to the form which appears tetragonal below the Curie point.

peratures unattractive even though theoretically possible. I

The calcining temperature must be below the firing temperature of the barium titanate ceramic, e. g. 1350 C.

At the firing temperature, the crystallites change to the:

form which appears tetragonal below the. Curie point, unless inhibited as presently described.

Upon adding to the calcined powder minute amounts of iron (or other metals) as disclosed inapplication Serial No. 224,389, the mixture is then conditioned for making a dense ceramic by usual techniques.

at the firing temperature. 1 The ceramic will have crystallites which appear cubic below the Curie point by X-ray diffraction. The eflect of the minute additions is to inhibit the growth of the crystallites during the firing of the ceramic at the firing temperature,-e. g. 1350 C. The

particle size of the calcined barium titanate produced as i above described is pigment or sub-pigment size and it is with these very small particle sizes that the iron is effectiye in preventing the growth of the barium titanate .crystallites to sizes which appear tetragonal instead of --of iron),.but the effect is thesame', namely, inhibiting cubic below the Curie point.

It has been observed that different batches of identical I chemical composition exhibit substantial variations in the properties of the fired ceramics. These difierences have been traced to variations in the stoichiometric ratio of the BaOTiOz in the barium titanate ceramic. For best results, the TiOz should be a fraction of a, per cent in excess of the stoichiometric ratio, e. g. as little as .l%' to 3%. Although the analytical methods now available frequently yield results expressed to .1%, the accuracy cannot'be depended upon to closer than-2%. When it is considered that as little as .1% excess TiOi is suflicient it will be seen that analytical methods of determining this excess are of not much help. It is simplerto determine the necessary excess of 'IiOz by firing test samples from the main batch to which-varying amounts of TiO: and BaO' have been added and then to treat the main batch in accordance with. results indicated by the test samples.

One possible theory for the excess TiO'a is that the excess TiOz reacts with the iron to form an iron titanate. which is isomorphous with the barium titanate. The excess TiOz or of BaO is of no importance at all if the desired end product is a barium titanate ceramic n which the crystallites appear tetragonal below the 01.1116 pomt.

It is only when the cubic'crystallites are desired that the excess TiOa assumes importance. It is true that an oce casional batchmay be made which will accidentally have Conveniently the iron can be in the form of iron oxide which breaks down the required excess TiOz to produce the cubic crystallites.

Almost all batches do require adjustment of the stoichiometric ratio and without knowing that the ad ustment was necessary and in what direction the adjustment should be made the results are variable.

What is claimed as new is: v r 1. The method of making barium titanate ceramic in which the crystallites appear cubic at temperatures below the Curie point as observed by X-ray diffraction which 7 comprises preparing a crystalline barium titanate salt of an organic acid, calcining the salt to drive oil the acid radical and to produce a fine powder'm which the crystallites appear cubic at temperatures below the. Curie point 1 -as observed by X-ray difiraction, adding minute amounts which comprises preparing a crystalline barium titanate salt of'an organic acid, calcining the salt to drive oil the acid radical and to produce a fine powder in which the crystallites appear cubic at temperatures below the )ur1e point as observed by X-ray difiraction, adding minute amounts of iron in the range of .1% to 3% by weight to the calcined powder to inhibit growth-during firing of the barium titanate crystallites to the form which appears tetragonal at temperatures below the Curie point as observed by X-ray difiraction, adjusting the stoichrometric ratio of the BaOTiOz to produce a fraction of a per cent excess TiOa, and firing the mixture to produce a dense ceramic in which the-crystallites appear cubic at temperatures below the Curie point.

3. The method of making a barium titanate ceramic in which the crystallites appear cubic at temperatures below the Curie point as observed by X-ray diffraction which comprises preparing a crystalline barium titanate oxalate, calcining the salt to drive oil the acid radical and to produce a fine powder in which the crystallites appear cubic at temperatures below the Curie point as observed by X-ray diffraction, adding minute amounts of iron in the range of .1% to 3% by weight to the calcined powder to inhibit growth during firing of the barium titanate crystallites to the form which appears tetragonal at temperatures below the Curie point as observed by X-ray difiraction,

and firing the mixture to produce a dense ceramic in which the crystallites appear cubic at temperatures belowthe Curie point.

4. The method of making a barium titanate ceramic in which the crystallites appear cubic at temperatures below the Curie point as observed by X-ray diffraction which comprises preparing a crystalline barium titanate oxalate, calcining the salt to drive off the acid radical and to produce a fine powder, in which the crystallites appear cubic at temperatures below the Curie point as-observed by X-ray dilfraction, 'adding minute amounts of iron in the range of .1% to 3% by weight to the calcined powder to 1 inhibit growth during firing of the barium titanate crystallites to the form which appears tetragonal at temperatures below the Curie point as observed by X-ray difiraction,

adjustingihe stoichiometric ratio of the BaOTiOz to produce ,a fraction of a per cent excess TiOzi, and firing the mixture to produce a dense ceramic in which the crystalli es appear cubic at temperatures below the Curie point as'observed by X-ray diffraction.

5. The method of claim 1 in which the minute amounts added to the calcined powder an iron in the rangeof .1 to 1 part by weight to parts of theBaTiOs.

6. The method of claim 1 in which the minute amounts added to the calcined powder are nickel in the range of .3 to 1 part by weight to 100parts of the BaTiOa.

7. The method of claim 1 in which the minute amounts added to the calcined powder are magnesium in the range of .3 to 1 part by weight to 100 parts of the BaTiOz.

8. The method of claim 1 in which the minute amounts added to the calcined powder are FeaOz, the weight of the iron in the FezOa being in the range of .1 to 1% of the weight of the BaTiOa.

9. The method of making barium titanate ceramic in which the crystallites appear cubic at temperatures below the Curie point as observed by X-ray diffraction which comprises preparing a crystalline barium titanate salt' of an organic acid, calcining the salt to drive off the acid radical and to produce a fine powder in which the crystallites appear cubic at temperatures below the Curiepoint as observed by X-ray diffraction, adding to the calcined powder in the range of .l% to 3% by weight metal.

selected from the group consisting of iron, nickel, cobalt, magnesium, calcium, and manganese to inhibit growth during firing of the barium titanate crystallites to the form which appears tetragonal at temperatures below the Curie point as observed by X-ray diffraction, and firing the mixture to produce a dense ceramic in which the crystallites appear cubic at temperatures below the Curie point.

10. The method of making barium titanate ceramic in which the crystallites appear cubic at temperatures below the Curie point as observed by X-ray diffraction which comprises preparing a crystalline barium titanate salt of an organic acid, calcining the salt to drive off the acid radical and to produce a fine powder in which the crystallites appear cubic at temperatures below the Curie point asobserved by X-ray diffraction, adding to the calcined powder in the range of .1% to 3% by weight metal selected from the group consisting of metalswhose' bivalent ionic radii are in the range of from 0.6 to 1.0

Angstrom unit and whose ionic potentials are in the range of from 1.4 to 1.8 to inhibit growth during firing of the barium titanate crystallites to the form which appears tetragonal at temperatures below the Curie point as o b-' served by X-ray diffraction, and firing the mixture to produce a dense ceramic in which the crystallites appear cubic at temperatures below the Curie point.

References Cited in the file of patent UNITED STATES P ATENTS Number Name Date 2,377,910 Wainer June 12, 1945 2,429,588 Thurnauer Oct. 21, 1947 2,436,839 Wainer Mar. 2,,1948 2,469,584 Wainer et al. May 10, 1949 2,538,554 Cherry Jan. 16, 1951 2,576,379 Woodcock et a1. Nov. 27, 1951 2,576,380 Woodcock Nov. 27, 1951 

9. THE METHOD OF MAKING BARIUM TITANATE CERAMIC IN WHICH THE CRYSTALLITES APPEAR CUBIC AT TEMPERATURS BELOW THE CURIE POINT AS OBSERVED BY X-RAY DIFFRACTION WHICH COMPRISES PREPARING A CRYSTALLINE BARIUM TITANATE SALT OF AN ORGANIC ACID, CALCINING THE SALT TO DRIVE OFF THE ACID RADICAL AND TO PRODUCE A FINE POWDER IN WHICH THE CRYSTALLITES APPEAR CUBIC AT TEMPERATURES BELOW THE CURRIE POINT AS OBSERVED BY X-RAY DIFFRACTION, ADDING TO THE CALCINED POWDER IN THE RANGE OF .1% TO 3% BY WEIGHT METAL SELECTGED FROM THE GROUP CONSISTING OF IRON, NICKEL, COBALT, MAGNESIUM, CALCIUM, AND MANGANESE TO INHIBIT GROWTH DURING FIRING OF THE BARIUM TITANATE CRYSTALLITES TO THE FORM WHICH APPEARS TETRAGONAL AT TEMPERATURES BELOW THE CURIE POINT AS OBSERVED BY X-RAY DIFFRACTION, AND FIRING THE MIXTURE TO PRODUCE A DENSE CERAMIC IN WHICH THE CRYSTALLITES APPEAR CUBIC AT TEMPERATURES BELOW THE CURIE POINT. 